1. Field of the Invention
The present invention relates generally to electronic circuits. More particularly, the present invention related to voltage peak detector circuits.
2. Description of Related Art
Peak detectors are used in various applications to provide an output voltage equal or representative of the peak voltage of an input signal. These peak detectors are used, by way of example only, in telecommunications applications in which a digital signal is received on a single twisted wire pair. Data must be extracted from the signal. A common method used to extract the data from the signal involves detecting the peak value of the incoming signal while sampling the incoming signal at mid-range intervals.
In U.S. Pat. No. 4,866,301 and U.S. Pat. No. 4,992,674 there is disclosed a peak detector that does not suffer certain disadvantages of peak detectors that came before it. This peak detector, which is discussed in detail below, does not, for example, track errant noise peaks like detectors that came before it, nor does it fail to respond rapidly to falling input signals.
Referring now to FIG. 1, there is shown a peak detector circuit such as that disclosed in U.S. Pat. Nos. 4,866,301 and 4,992,674. This peak detector circuit, generally designated by the reference numeral 10, includes a comparator 12 in which the input signal V.sub.IN is connected to the negative input. The output of the comparator 12 is connected to the gate of a p-channel transistor 14 and also to the gate of an n-channel transistor 16. The source of the p-channel transistor 14 is connected to a charging current source 18, the other end of which is connected to VDD. The source of the n-channel transistor 16 is connected to one end of a discharge current source 20, the other end of which is connected to ground. The drains of the p-channel transistor 14 and the n-channel transistor 16 are connected together and to one end of a holding capacitor 22, the other end of which is connected to ground. The common drain connection of the p-channel transistor 14 and the n-channel transistor 16 is also connected to the gate of an n-channel buffer or isolation transistor 24. The drain of the n-channel buffer transistor 24 is connected to VDD and the source is connected to one end of a third current source 26, the other end of which is connected to ground. The source of the n-channel buffer transistor 24 is also connected to the positive input of the comparator 12 and, further, it forms the peak voltage output signal shown as V.sub.PEAK in FIG. 1.
Operation of the peak detector circuit 10 was described in U.S. Pat. Nos. 4,866,301 and 4,992,674 as follows:
1. When the input voltage V.sub.IN is greater than the peak output voltage, V.sub.PEAK, then the output of the comparator 12 will be at a low voltage level such that the p-channel transistor 14 is conductive and the n-channel transistor 16 is nonconductive. When the p-channel transistor 14 is conductive, then current from the current source 18 puts charge onto the holding capacitor 22 to cause the voltage across the holding capacitor 22 to rise. This voltage on the holding capacitor 22 is decreased by the threshold voltage of the n-channel buffer transistor 24 to form the peak output voltage V.sub.PEAK.
2. When the input signal V.sub.IN is less than the peak output voltage V.sub.PEAK, then the output of the comparator 12 is at a high voltage level such that the p-channel transistor 14 is nonconductive and the n-channel transistor 16 is conductive. Under these conditions the charge on the holding capacitor 22 is discharged through the discharge current source 20 and the voltage across the holding capacitor 22 and, therefore, the voltage at the peak voltage output signal V.sub.PEAK is decreasing.
Although the peak detector disclosed in U.S. Pat. Nos. 4,866,301 and 4,992,674 possesses a number of advantages relative to its prior art, it is not without its shortcomings. For example, it cannot react properly to differential alternate mark inversion ("AMI") coding schemes, that is schemes in which zeros are coded as no pulses and ones are coded as alternative differential positive and negative pulses. That is to say, the peak detector disclosed in U.S. Pat. Nos. 4,866,301 and 4,992,674 cannot properly react to a situation where a differential positive pulse is followed by a differential negative pulse in tracking an AMI signal.
Based upon the foregoing, it should be understood and appreciated that it is a shortcoming and deficiency of the prior art that there has not heretofore been developed a peak detector that can track a fully differential input.